1. Field of the Invention
The base objective of the invention appears from the title. The term transversely oriented is here meant to include any orientation forming a significantly large angle with the longitudinal direction of the film including but not limited to orientations which are perpendicular or almost perpendicular to the longitudinal direction. The main purpose of making transversely oriented film is as part of the manufacturing process for crosslaminates, i.e., laminates of films of which at least two are uniaxially or unbalanced biaxially oriented, and in which the main direction of orientation in one of these films crosses the main direction in the other one.
2. Description of the Related Art
As background for the understanding of the invention a more detailed view over existing crosslamination technology may here be useful. This mainly concerns the inventor's earlier publications.
The polymer materials for crosslaminates have been mainly and are mainly polyethylene and polypropylene of different types often modified by blending, and the old and present industrialized manufacturing processes comprise the steps of extruding a tube, which, by the draw-down, is oriented mainly in its longitudinal direction, helically cutting this tube to a web with its main direction of orientation on the bias, and continuously laminating two or more such webs with their main directions of orientation criss-crossing. There can also as mentioned be included in the laminate a film which is oriented mainly in its longitudinal direction.
In the first commercialized technology based on these principles, the extruded tubular film, which is melt-oriented mainly in its longitudinal direction, is further cold stretched in this direction prior to the helical cutting. In a later commercialized technology, disclosed, e.g., in U.S. Pat. No. 4,039,364, each tubular film is coextruded, having a layer which contributes mainly to the tensile strength in the laminate and at least one surface layer adapted to help in the bonding of the films, which at least partly takes place by pressure and heat.
Also special layers are coextruded on the films, which become exterior in the laminate. These special layers are adapted to modify the surface properties of the laminate, especially for improved heat-sealing. In this later technology the helical cutting takes place in direct succession to the coextrusion without any cold stretching between, but in a separate production line. Further stretching is carried out when the films have been brought together in a sandwich arrangement, bonded or not yet bonded, to form a laminate. The films are biaxially stretched at a relatively low temperature. The transverse component of this biaxial stretching takes place between grooved rollers. In U.S. Pat. No. 5,028,289 and U.S. Pat. No. 5,626,944 this stretching between grooved rollers has been further developed.
Practical ways of carrying out the helical cutting are disclosed in U.S. Pat. No. 5,248,366. This patent also mentions an alternative cutting technique, namely that the tubular film can be provided with a helically extending melt orientation while it is drawn off from the coextrusion die, established by a relative rotation between the exit of the die and the draw-down means, and subsequently the cutting may be parallel with the axis or may be at an angle to the main direction of orientation. The process may even be adjusted to produce a web in which the main direction of the melt orientation will become perpendicular to the longitudinal direction of the web.
For the sake of completeness it should be mentioned that, in very early patents, there is also disclosed the possibility that longitudinally oriented polymer film material can be discontinuously crosslaminated and bonded in a press.
In a process which is entirely different from that described above, crosslaminates of a very stiff character are made for use in special advanced products. They consist of polymers which in molten or part-molten state are liquid crystals, and which become oriented and crosslaminated already within the extrusion die by means of counter-rotating die-parts. However, this type of process and product is not a subject of the present invention.
Reverting to the other types of crosslaminates, which more are commodities or technical products, the heat-seal strength in a shear-type seal is adequate when a suitable lower melting polymer has been chosen for the surface layers of the laminate, while very special precautions must be taken if good shock-heat-seal strength is requested in peel-type heat-seals, as usually needed for industrial bags supplied with such heat-seals. These precautions are disclosed in U.S. Pat. No. 5,205,650 and WO-A-98/23434.
Recent inventions concerning crosslaminates comprise the inventor's five publications WO02/102592, WO04/54793, WO03/033241, WO04/094129 and WO05/102669. The first two supply one or both plies in a 2-ply crosslaminate with a waved structure like the waving in corrugated paper board, but generally with a wavelength which is shorter than normal for the paper board, generally with the waves extending in the direction of molecular orientation of the respective ply. It is noted that the present invention is useful to manufacture of a transversely oriented film for these crosslaminates.
Both of these publications disclose and claim the formation of a pattern of thinner, biaxial webs. In WO02/102592 it concerns only one of the plies and is described throughout the specification from page 8 onwards and in the claims, and in WO04/054793 it concerns one or both plies and is described throughout the specification from page 12 line 19 and in the claims. In both cases it concerns one set of thin linear biaxially oriented webs, which essentially form bonded base of each flute, and further one set of narrower, thin linear biaxially oriented webs, each forming the crest of a flute.
WO03/033241 and WO04/094129 which have been mentioned above, concern in particular special attenuation and orientation processes in connection with the extrusion, by which the strength at elevated temperature, heat-seal properties, yield tension and/or barrier properties can be improved.